A reluctance type motor has so numerous disadvantages including vibration that few reluctance type motor has ever been practically utilized, although its output torque is large and no magnet rotor is required. Attaining both size reduction and speedup of motor rotation is very difficult to practically realize. Although rotational torque is obtained by magnetic attraction force acting between magnetic poles of the fixed armature and salient poles of the rotor, the magnetic attraction force is directed toward a center thereof. Thus, mechanical vibration is generated.
Such conventional motors require armature coils that are controlled by switching elements connected to both ends thereof for activating or deactivating these armature coils. Hence, expensive power elements increase in number and, therefore, the cost increases.
Furthermore, switching elements provided at a positive terminal side of an electric power source tend to be expensive as they require input electric signals supplied from another electric power source for controlling currents supplied to the armature coils.
A reluctance type motor has a rotor equipped with numerous salient poles and, therefore, its inductance is large. This increases magnetic energy amount stored into or discharged from magnetic poles or salient poles. And also increased is repetition frequency of such energy storage and discharge during one complete revolution of the rotor. It is, therefore, a problem that the reluctance type motor cannot rotate in a high-speed region nevertheless its large output torque. Also, it is another problem that the size of the reluctance type motor cannot be reduced due to a large number of salient poles.
Here, a low speed should be considered to be around 300 r.p.m and a high speed around 60 thousands rpm.
If compared with a DC motor having a magnet rotor, an extraordinary large inductance of the armature coil will cause a slow building-up of exciting current at an initial stage of the current supply period, as well as a slow trailing-edge at a terminating stage of the current supply period. The former will cause a smaller output torque, and the latter cause a counter torque.
In order to make building-up of armature current sharp in the initial stage of the current supply period, a voltage of an electric power source is increased. However, such building-up of the armature current will be too much sharp in a region after the magnetic saturation point. For this reason, the motor causes vibrations and electric noises. And, as above-described building-up section of the armature current corresponds to a section where the torque is small, only disadvantages will be enhanced. Thus, there is a problem such that a high-speed rotation cannot be realized due to the above-described torque reduction and counter torque. As the number of the salient poles is too much, magnetic energy is so numerous times transferred between armature coils. This increases iron loss. Accordingly, there is a problem that efficiency is lowered in a high-speed region.
If the applied voltage is increased in order to speed up the rotational speed, more than 600 volts will be required. This means no practical motor will be obtained.
If the building-up and trailing-edge of the armature current are sharpened in order to realize the speedup of the motor, iron loss will be correspondingly increased. It will be ideal to use a half-wave of sine waveform. This will not be easily obtained for some reasons.
Furthermore, salient poles of the rotor are generally required at least 4. Therefore, repetition frequency of energy storage and discharge during one complete revolution of the rotor will increase between the magnetic poles and the salient poles. Thus, efficiency is lowered and it becomes difficult to speed up the motor speed.
Moreover, a large magnetic attraction force, which does not contribute the torque generation, is generated between the magnetic poles and salient poles. Although magnetic poles are provided symmetrically about an axis in order to eliminate this force, this magnetic attraction force is not completely erased because of difference of air gap length between the magnetic poles and salient poles. This becomes a cause of vibration. Even if the air gap length is finely adjusted, it will be soon changed due to wear occurring during use of motor.
FIG. 26 shows a plan view showing a well-known three-phase half-wave current supply mode reluctance type motor. A reference numeral 16 represents a fixed armature which is made of well-known laminated silicon steel sheets. Magnetic poles 16a, 16b,--are associated with armature coils 17a, 17b,--. A rotor 1 rotates in a direction of an arrow A. A reference numeral 5 represents a rotational shaft 5. When armature coils 17b, 17e are activated, the rotor 1 rotates in the direction of the arrow A. After 120-degree rotation, these armature coils are deactivated. Next, armature coils 17c, 17f are activated. After 120-degree rotation, these armature coils are deactivated.
As described above, the rotor 1 rotates in the order of the armature coils 17a, 17d--17b, 17e--17c, 17f along the arrow A. Only two salient poles contribute the generation of above-described rotational torque, and remaining four salient poles have no relation to this generation of rotational torque. If all the six salient poles generate torque simultaneously, a generated torque will be three times. However, this is not attainable.
Furthermore, when the armature coils 17a, 17d are activated, the magnetic poles 16a, 16d are magnetically attracted radially toward the salient poles 1a, 1e. Thus, the fixed armature 16 suffers deformation due to this attraction force. When the rotor rotates, the fixed armature suffers deformation due to another attraction forces generated by the magnetic poles 16b, 16e and 16c, 16f and their confronting salient poles. These deformation mechanism induces the vibration of the motor.
As it is technically difficult to equalize air gap length between the salient poles and magnetic poles, an attraction force received by the rotor 1 changes its direction as the rotor 1 rotates. Thus, the rotor 1 causes vibration in the radial direction. Accordingly, vibration noise is generated. And, the durability of the bearing, provided for the rotational shaft of the rotor 1, is worsened. In case of a large-output motor, the above-described problems cannot be solved.
Accordingly, the present invention has an object to provide a reluctance-type motor which is small in diameter size and having good efficiency in a high-speed region, and also to provide a reluctance-type motor which is capable of generating a large output torque, suppressing vibration, and performing regenerative braking as occasion demands.